This study investigates cellular tropism of the human polyomavirus, JC virus (JCV) and the cellular events leading to induced brain pathology in humans and animal models of JCV infection. JCV remains latent following initial childhood infection, and reactivation of the virus in immunocompromised individuals causes the fatal CNS demyelinating disease, progress multifocal leukoencephalopathy (PML). When injected intracerebrally into newborn hamster, JCV establishes a nonproductive infection inducing brain tumor formation. In human PML, JCV infection of oligodendrocytes leads to cell death and subsequent demyelination. Astrocytes are also infected by JCV but the role of this cell in initial brain infection and disease progression is less well understood. Astrocytic infection by JCV as well as the possible involvement of other cell types in disease pathogenesis is being examined in: (1) human PML tissues, and (2) animal models of JCV brain infection. Current efforts have focused on developing and improving immunostaining techniques to recover antigen expression in formalin-fixed paraffin-embedded PML brain tissue as well as devising double-label methods to identify JCV-infected cells in human and hamster brains. Improved staining sensitivity has revealed positive astrocytes in close association with blood vessels in PML brain. Additional viral expression was detected in GFAP negative perivascular cells. In contrast to these findings, JCV does not appear to infect astrocytes in the hamster model. Instead JCV primarily infects and induces neoplasia in granule neurons. Increased GFAP expression and reactive gliosis is evident in areas adjacent to neoplastic cells. In addition, JCV infects vascular endothelial cells in the hamster brain. These findings suggest that these cell types may possibly be involved in human disease and preliminary studies investigating endothelial cell involvement and blood~brain barrier alteration in the hamster model and human PML are in progress.